The present invention relates generally to hydraulic brakes. More particularly, the present invention relates to hydraulic brakes that provide both parking brake and service brake functions.
Braking devices having combined service brakes and parking brakes are commonly used on vehicles such as mobile booms, pallet loaders, etc. FIGS. 1 and 2 illustrate one type of prior art braking device 20 capable of functioning as both a parking brake and a service brake. The braking device 20 includes a rotational shaft 26 that extends through a plurality of co-axially aligned rotor and stator plates 22 and 24. A first end 28 of the shaft is adapted for connection to a wheel, while a second end 30 of the shaft is adapted for connection to a drive source such as a hydraulic motor. The rotor plates 22 are adapted to rotate in concert with the shaft 26, while the stator plate 24 are prevented from rotating in concert with the shaft 26. The plates 22, 24 are enclosed within a housing 23.
The plates 22, 24 of the braking device 20 are arranged in a stack 21 having a first axial end 32 positioned opposite from a second axial end 34. A spring assembly 36 is positioned at the first axial end 32 of the stack 21. The spring assembly 36 provides a spring load for biasing the rotor and stator plates 22, 24 together to provide the parking brake. When the parking brake is engaged, the plates 22, 24 are compressed between the spring assembly 36 and the housing 23 such that friction between the plates 22, 24 prevents the shaft 26 from freely rotating.
The parking brake is disengaged by a first piston 38 positioned at the second axial end 34 of the stack 21. An axial extension 40 is provided for transferring force from the first piston 38 to a parking brake release plate 39 engaged by the spring assembly 36. When hydraulic pressure is applied to the first piston 38, the first piston 38 provides a force that overcomes the bias of the spring assembly 36 and moves the parking brake release plate 39 away from the remainder of the plates 22, 24. With the spring load removed from the plate stack 21, the plates 22, 24 are no longer compressed together and the parking brake is released.
The braking device 20 also includes a second piston 42 for actuating the service brake of the braking device 20. The second piston 42 is mounted at the first axial end 32 of the stack 21. When the service brake is actuated as shown in FIG. 2, hydraulic pressure applied to the second piston 42 causes the second piston 42 to compress the plate stack 21. To engage the plate stack 21, the second piston 42 travels through an opening 37 defined by the parking brake release plate 29. With the stack 21 compressed as shown in FIG. 2, friction between the plates 22, 24 provides a frictional braking action to the shaft 26.
The hydraulic brake industry is very competitive and cost is an important factor relating to the success or failure of any given product. As shown by the braking device of FIGS. 1 and 2, conventional combined parking and service brakes typically are fairly complicated. Therefore, such brakes are typically fairly expensive to manufacture. What is needed is a simplified and cost effective braking device that functions as both a parking brake and a service brake.
One aspect of the present invention relates to a braking device having a service brake and a parking brake. The braking device includes a housing and a shaft that extends at least partially through the housing. The shaft is rotatable relative to the housing about a longitudinal axis. The braking device also includes a stack of plates spaced axially along the shaft. At least one of the plates is adapted to rotate in concert with the shaft. The stack of plates includes a first axial end positioned opposite from a second axial end. A spring structure is provided for compressing the stack of plates together to provide the parking brake. The spring structure is mounted at the first axial end of the stack of plates. The braking device further includes a first piston for releasing the parking brake, and a second piston for actuating the service brake. The second piston is mounted at the second axial end of the stack of plates. In use, the second piston forces the plates axially toward the spring assembly to actuate the service brake.
Another aspect of the present invention relates to a method for actuating a service brake of a braking device that also includes a parking brake. The braking device includes a shaft that is rotatable about a longitudinal axis and is adapted for connection to a wheel. The braking device also includes a stack of plates spaced axially along the shaft with at least one of the plates being adapted to rotate in concert with the shaft. The braking device further includes a spring structure for biasing the plates together to provide the parking brake. The service brake of the braking device is actuated by forcing the stack of plates axially toward the spring structure thereby causing the plates to be compressed together to provide the service brake.